Diesel engines and other lean-burn engines or power plants are operated at higher than stoichiometric air to fuel mass ratios for improved fuel economy. Such lean-burning engines produce a hot exhaust with a relatively high content of oxygen and nitrogen oxides (NOx). The temperature of the exhaust from a warmed-up diesel engine is typically in the range of 200° C. to 400° C. and has a representative composition, by volume, of about 10–17% oxygen, 3% carbon dioxide, 0.1% carbon monoxide, 180 ppm hydrocarbons, 235 ppm NOx and the balance nitrogen and water. These NOx gases, typically comprising nitric oxide (NO) and nitrogen dioxide (NO2), are difficult to reduce to nitrogen (N2) because of the high oxygen (O2) content in the hot exhaust stream.
One approach for NOx reduction in lean-burn engine exhausts is by injecting urea into the exhaust and passing the stream over a suitable reduction catalyst. Urea decomposes in the hot exhaust producing NH3 that reacts with NOx to produce N2 and H2O. The practice is called selective catalytic reduction by urea (urea/SCR). But a drawback of the approach is that urea must be carried onboard the lean-burn engine powered vehicle.
Co-pending U.S. patent application Ser. No. 10/864,717, assigned to the same assignee as this invention, discloses a method of separately adding ozone and nonthermal plasma-reformed diesel fuel constituents to the exhaust stream flowing from the engine or power plant preparatory to selective catalytic reduction of NOx. Ozone is added to the exhaust stream for oxidation of NO to NO2. And plasma-generated, low molecular weight oxygenated hydrocarbons and hydrocarbons from a fractionated portion of the diesel fuel hydrocarbon mixture are added to the exhaust as reactants for conversion of NO2 to N2 over the reduction catalyst. This process can be called diesel fuel/SCR.
The present specification describes a further and related improvement in non-thermal plasma processing of diesel fuel for reduction of NO and NO2 in exhaust streams from lean-burn engines and power plants.